Tiltrotor aircraft typically include multiple rotor assemblies that are carried by the wing member of the aircraft and are generally disposed near the end portions of the wing member. Each rotor assembly may include an engine and transmission that provide torque and rotational energy to a drive shaft that rotates a proprotor including a plurality of proprotor blade assemblies. Typically, the rotor assemblies are capable of moving or rotating relative to the wing member enabling the tiltrotor aircraft to operate between a helicopter mode, in which the rotor assemblies are tilted upward, such that the tiltrotor aircraft flies much like a conventional helicopter and an airplane mode, in which the rotor assemblies are tilted forward, such that the tiltrotor aircraft flies much like a conventional propeller driven aircraft. In this manner, the proprotors generate greater lift in helicopter mode than in airplane mode, as the proprotors are oriented to generate greater thrust propelling the aircraft vertically. Likewise, the proprotors generate greater forward speed in airplane mode than in helicopter mode, as the proprotors are oriented to generate greater thrust propelling the aircraft forward.
In conventional tiltrotor aircraft, the proprotor blade assemblies include a D-shaped spar having the bulk of its structure biased forward, offering favorable structural coupling and mass properties. Unfortunately, D-shaped spars are structurally complex having tight internal radii and complicated material transitions resulting in a component that is particularly difficult, time consuming and expensive to manufacture. Accordingly, a need has arisen for an improved proprotor blade assembly having a spar with a less complex structure that is simpler, less expensive and less time consuming to manufacture.